The present invention relates to an improvement of a harmonics generation element for use, as a light source of short wave length or the like, in the field of optical recording or the like. In particular, the present invention relates to a harmonics generation element improved as to the properties of single-crystals of lithium niobate to be used in such a harmonics generation element.
Higher speed as well as higher density are demanded for information processing, such as information transmission, information recording, information reproduction, and the like, in the field of information processing techniques. Optical techniques, such as optical recording and the like, have begun to be used in an attempt to satisfy the demand. It can be presumed that the tendency to increase the speed and density will be more accelerated in the future. To this end, it is thought that required is a light source which has a shorter wavelength (for example, in the range of from about 400 nm to about 700 nm), which has output power in the range of from the order of mW to the order of tens of mW in practical use, and which has a high response.
For such a light source, two techniques have been investigated. One is a laser diode using a Group III - Group V or Group II - Group IV compound semiconductor. The other is a harmonics converter such as a secondary harmonics generator, a tertiary harmonics generator, or the like. The former is now under research and the most favorable results thus far are a laser diode having a wavelength of 700 nm. Accordingly, long-term research will be required for development of a practical laser diode in the point of view of output beam power as well as shortening of the wavelength. length. It is thought that the wavelength of a light source using such a semiconductor laser may be limited to 400 nm. On the other hand, SHG (second harmonic generation) elements using crystals of lithium niobate (for example, as described in an article, "LiNbO.sub.3 --Waveguide Element for Second Harmonic Generation in Laser with 25% Conversion Efficiency", NIKKEI ELECTRONICS No. 399, issued Jul. 14, 1986, pages 89 to 90) are known as one of the latter technique. In recent years, the SHG elements have been available in the market so as to be put into practice.
However, a problem in the field of optical applied techniques using such an SHG element is in that the SHG conversion efficiency of materials used for the SHG element is low. Although the conversion efficiency varies according to the incident beam power, the present situation is that the conversion efficiency is only 1 or 2 percent in the case where a waveguide type element using crystals of lithium niobate is used.
In the field of optical communication using long wavelength light sources having a wavelength of 1.3 .mu.m or 1.55 .mu.m, for example, in the case where crystals of lithium niobate are used in optical modulators and the like, occurrence of optical damage becomes no practically serious problem if the energy strength of the light sources is not greater than about 10 mW. On the other hand, in the case where short wavelength light is used for the purposes of optical recording and the like, occurrence of optical damage becomes a serious problem even though the energy strength of the light sources is about 0.1 mW. Therefore, various proposals have been made. One of the proposals is a method of reducing optical damage through addition of magnesium (as reported in a paper by D. A. Bryan et al., "Increased Optical damage resistance in lithium niobate", Appl. Phys. Lett., vol. 44, No. 9, page 847, 1984).
According to the Bryan et al. report, it is known that optical damage resistance in crystals of lithium niobate increases as the magnesium content of the crystals increases, and that the optical damage resistance takes a constant value after the magnesium content reaches 5 atomic percent. Accordingly, in the case where crystals of magnesium-containing lithium niobate are used in optical elements sensitive to optical damage, it is common knowledge that the magnesium content is made to be not less than 5 atomic percent (or 5 mol percent as MgO).
In general, optical damage becomes more pronounced as the wavelength related to the optical damage becomes shorter. Accordingly, it is predicted that optical damage will become a more serious problem in materials for harmonic conversion elements used for light of a wavelength shorter than the 1.55 .mu.m wavelength light currently used in optical communication and the like.
As described above, single-crystals of lithium niobate have begun to be put into practice as a material for harmonics generation elements. However, there is the problem of low conversion efficiency. In addition, it is established that reduction of optical damage is required in practical use.